The present disclosure relates generally to gas turbine engines and, more particularly, to power generation systems that operate with an alternative working fluid.
Gas turbine engines produce mechanical energy using a working fluid supplied to the engines. More specifically, in known gas turbine engines, the working fluid is air that is compressed and delivered, along with fuel and oxygen, to a combustor, wherein the fuel-air mixture is ignited. As the fuel-air mixture burns, its energy is released into the working fluid as heat. The temperature rise causes a corresponding increase in the pressure of the working fluid, and following combustion, the working fluid expands as it is discharged from the combustor downstream towards at least one turbine. As the working fluid flows past each turbine, the turbine is rotated and converts the heat energy to mechanical energy in the form of thrust or shaft power.
Air pollution concerns worldwide have led to stricter emissions standards both domestically and internationally. Pollutant emissions from at least some gas turbines are subject to Environmental Protection Agency (EPA) standards that regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO). In general, engine emissions fall into two classes: those formed because of high flame temperatures (NOx), and those formed because of low flame temperatures that do not allow the fuel-air reaction to proceed to completion (HC & CO).
Air has been used as a working fluid because it is readily available, free, and has predictable compressibility, heat capacity, and reactivity (oxygen content) properties. However, because of the high percentage of nitrogen in air, during the combustion process, nitrogen oxide (NOx) may be formed. In addition, carbon contained in the fuel may combine with oxygen contained in the air to form carbon monoxide (CO) and/or carbon dioxide (CO2).
To facilitate reducing NOx emissions, at least some known gas turbine engines operate with reduced combustion temperatures and/or Selective Catalytic Reduction (SCR) equipment. However, operating at reduced combustion temperatures reduces the overall efficiency of the gas turbine engine. Moreover, any benefits gained through using known SCR equipment may be outweighed by the cost of the equipment and/or the cost of disposing the NOx. Similarly, to facilitate reducing CO and/or CO2 emissions, at least some known gas turbine engines channel turbine exhaust through a gas separation unit to separate CO2 from N2, the major component when using air as the working fluid, and at least one sequestration compressor. Again however, the benefits gained through the use of such equipment may be outweighed by the costs of the equipment.